Stranica 18 [18]
OCR
i. Reflected images The factors affecting reproducibility and device dependence of reflected images are discussed in the sections below. Three main effects are considered: 1) Spatial inhomogeneities of the radiation source; 2) Spectral density of the radiation source and non-linear camera response; 3) Camera response (colour calibration for visible-reflected images only). In addition, the need for image registration of the various reflected images in order to produce false-colour images is discussed. — Spatial inhomogeneities of the radiation source The spatial distribution of radiation reflected from objects and captured by a camera depends, among other factors, on the position of the radiation source(s) and the geometry of the object. Non-uniform illumination can lead to spatial inhomogeneities introduced by the illumination system. Figure 1-6(a) and (c) show two images of a uniformly reflective board taken under different illumination conditions: Visible and UV illumination, respectively. Although these images look relatively uniform, it is clear from the enhanced contrast images (iso-grey level values) shown in Figure 1-6(b) and (d) that the distribution of incident radiation in each case is very uneven. Visible-IR illumination UV illumination WB visible: Enhanced WB UV Enhanced min RGB = 126 contrast Min RGB = 160 contrast max RGB = 198 max RGB = 231 A= 28% A= 28% Figure 1-6. Images of a uniformly reflective board taken under different illumination conditions: (a) Visible and (c) UV illumination. Images (b) and (d) are enhanced contrast images showing the isogrey levels corresponding to the distribution of incident visible light in (a) and UV radiation in (b) on the surface under investigation. " An example of the impact of non-uniform illumination on the resultant reflected image is shown in Figure 1-7(a), where the upper section of a visible-reflected image appears to be Version No. 1.0 10 Date : 14/10/2013 NTH FRAMEWORK
